You'll need a 14 mm socket to take off the crankarms nuts and a crankarm puller to get access to the bottom bracket. If it's a sealed bb and the cups are tight (you'll need the appropriate cup tool/ depends on bb manufacture) then it's shot and you will need a new one. A cup and ball bearing one can be adjusted as long as there is no internal damage, you would need a lockring tool and a pin spanner to it, though it could be hacked with crude tools. It might be cheaper and easier to have a bike shop do it.

Are you sure it's not just the crankarm nuts that are loose? That will cause a loud creaking noise during each pedal stroke.

Pull off those grey plastic caps in the center of the cranks with a small screwdriver or pick and check to see if the nuts under them are tight. There is a proper torque specification for those but really tight will do, better then leaving them loose if that is the case.

Make sure it's the bottom bracket and not the pedals before you spend money. IME, it's a lot easier to wear out a pedal bearing.

If you drop the chain onto the bottom bracket shell so you can turn the crank without turning the rear wheel, you can probably tell just by turning the crank and then the pedals individually with your fingers - you'll actually feel the bearings grinding.

If it is the bottom bracket, consider a whole new crankset. It'll be more expensive now but you'll be able to replace the chain rings when they wear out. I just hate to spend money supporting a disposable part.

Depending on how much you can spend, how nice of parts you want, and the level of service you're looking for, there's your Local Bike Shop, maybe a community bike shop or coop, and The Internet. On the 'net, the best prices going tend to be nashbar.com, performancebike.com, pricepoint.com, sometimes backcountryoutlet.com. Also check Amazon and Google Shopping.